Adding large capacitors to the power lines involves some risks: During startup the empty capacitor draws very high current which may cause damage to switching power supplies (depending on their design) and in the long run also to the capacitors. So don't overdo it - maybe only one capacitor / less capacity is enough to improve the gameplay. Here are some explanations for those who are more deeply interesed: http://www.ti.com/lit/an/slva670a/slva670a.pdf

I also like the idea - just wanted to make you aware of the topic. Most switching power supplies (SPS) can handle overloads: Some limit the current, others shut down and immediately restart. And some even include a soft start. It would help to know the specs of the 70 Volt SPS, but i couldn't identfy the model. BTW the 12 Volt SPS in my Alien is this one: http://www.deltapsu.com/products/panel-mount-power-supply/PMC-12V150W1BA (mentioning "Start-up with Capacitive Loads: 8,000µF Max" and "Hicc-up Mode" as overcurrent protection). I might also have a look at a solution to limit the inrush current, but the high voltage is challenging.

Did you recognize that the cabinet board is prepared for the components of a transformer based solution: voltage selection, bridge rectifier etc.?

Wow, that would really be some kind of "dirty hack" to limit currents with undersized wire. I already asked myself if Heighway ran out of acceptable wire and planned the replacement together with the ridiculous thin 12V cables to the mainboard. But with the caps there should be even more blown mosfets (if it is current related)?

I have to admit, that exactly these challenges are part of the "fun" I experience with my alien. I like to analyse and repair/improve things. I own my blue LE since 4 weeks and never played a single game . Currently I am installing a playfield protector which keeps me busy.

My 70V switching power supply is marked to be a "Weho S-500-70" (70 Volt, 500W or 7 Amps). The S-500 series is listed here http://wehopower.com/products/s-500-ID29.html, but the 70 Volt version seems to be a custom order. Any switcher with similar values should work as an replacement.
Mean Well power supplies are well known for their reliability, but the standard offer also does not include a 70V model: https://www.meanwell.com/productPdf.aspx?i=469

In fact very early Aliens use the same toroidal transformer as the FT. It deliveres 50VAC (resulting in approx. 65 VDC) at 6 Amp: http://www.antekinc.com/as-3450-300va-50v-transformer/ (the 15 Volt output was never used in Aliens). But the increased number of coils draw higher currents, so they replaced the transformer by the Switcher and disabled some components on the green power supply board. I am convinced that a 50VAC, 10 Amp transformer would also perfectly - if not better - do the job (with some additional modifications).
FT Transformer (resized).jpg

Tom,
the LED chips are connected in chains. If one chip of a chain fails, all following will get no data anymore.
The sling LEDs are all in the same chain B of I/O Board 3 (see http://pinwiki.com/wiki/index.php?title=Heighway_Pinball_Repair):
3-B7 GI Right Sling Bottom
3-B8 GI Right Sling Top
3-B9 GI Left Sling Bottom
3-B10 GI Left Sling Top
So if B7 (right side bottom) fails, all other sling LEDs will stay dark. During your test you replaced (by luck) the defect LED - so all were working again.

I had the same problem with the double LED boards from TimeBandit: one LED almost immediatedly stopped working. I replaced it and since then everythings looks good.
Those LED Chips are really cheap stuff - You can buy 50 of them for 3$ inluding shipment. So I am not surprised about the short life of (hopefully only) some of them.
LG Per